The present invention relates generally to ink jet printers, and more particularly to a method and apparatus for improving the performance of continuous stream ink jet printers which deflect ink droplets through asymmetric heating thereof.
Traditionally, color ink jet printing is accomplished by one of two technologies referred to as xe2x80x9cdrop-on-demandxe2x80x9d and xe2x80x9ccontinuous streamxe2x80x9d printing. In each case, ink is fed through channels formed in a printhead. Each channel includes a nozzle from which droplets of ink are ejected and deposited upon a medium. Typically, each technology requires separate ink supply and delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce up to several million perceived color combinations.
In drop-on-demand ink jet printing, ink droplets are selectively ejected for impact upon a print medium using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the printhead and the print medium and strikes the print medium. The formation of printed images is achieved by controlling the individual formation of ink droplets as the medium is moved relative to the printhead. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Typically, either heat actuators or piezoelectric actuators are used as pressurization actuators. With heat actuators, a heater heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric potential is applied to a piezoelectric material possessing properties that create a pulse of mechanical movement stress in the material causing an ink droplet to be expelled by a pumping action. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuous ink jetxe2x80x9d printing, uses a pressurized ink source for producing a continuous stream of ink droplets. The droplets are then selectively deflected to either strike the print medium or not. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism. Typically, continuous ink jet printing devices are faster than droplet on demand devices.
U.S. Pat. No. 6,079,821 discloses a continuous stream ink jet printer in which periodic heat pulses are applied to the ink filament to break the filament into droplets. Droplets can be deflected, either into a reservoir or onto a print medium by selective actuation of one or more of plural heater sections disposed around an ejection nozzle. In other words, selective deflection is accomplished by asymmetrically heating the ink droplets to create a temperature gradient within the droplets.
Asymmetrically applied heat results in droplet deflection having a magnitude, i.e. angle, that depends on several factors. For example, the geometric and thermal properties of the nozzle, the quantity and differential of applied heat, the ink pressure, and thermal properties of the ink all affect deflection angle. Of course, the greater the deflection angle of the ink drops, the more reliable, compact, and accurate the printer can be. The thermal properties of ink can be adjusted to some extent. However, in order to maintain compatibility with a plurality of available inks, it is desirable for a printer to be capable of using standard ink compositions. Also, it is difficult to impart a great deal of heat to the ink stream in an asymmetrical manner, i.e., to create a large temperature gradient, because of the relatively high rate of heat conduction in the ink and the relatively small dimensions of typical ink flow channels and nozzles. Accordingly, complex heater and nozzle arrangements have been developed to improve deflection angles of ink droplets in continuous stream printers.
Commonly assigned U.S. Pat. No. 6,247,801 discloses an arrangement for asymmetric heating of ink droplets in continuous ink jet printers.
It is an object of the invention to improve printing consistency in an ink jet printer. To achieve this object and other objects, a first aspect of the invention is a continuous stream ink jet printer, comprising a printhead having at least one nozzle having an axis for continuously ejecting a stream of ink droplets an ink supply for providing liquid ink to the printhead, a heater disposed adjacent the nozzle for generating heat that thermally deflects selected ink droplets at an angle with respect to the axis to effect a printing operation, and a cooling unit for cooling ink provided to the printhead to increase the deflection angle of the droplets.
A second aspect of the invention is a method of printing with a continuous ink jet printer comprising cooling ink to a temperature lower than an ambient temperature, ejecting the ink as a filament out of a nozzle along an axis, breaking the filament up into droplets, and wherein the ink is asymmetrically heated to selectively deflect the droplets off of the axis.